The present invention is related to a non-instantaneous interrupt handover method and an apparatus capable of realizing such a non-instantaneous interrupt handover in a mobile communication system.
In mobile communication systems with using the CDMA (code division multiple access) system, non-instantaneous handover is realized. For instance, wireless (radio) interfaces are defined based upon the EIA/TIA/IS-95 standard as the portable telephone standard system in USA. Similarly, this EIA/TIA/IS-634 defines interfaces between base stations and a mobile switching center.
In these mobile communication systems, the soft handoff or soft handover techniques have been employed as the service area switching techniques during communications. In this soft handover technique, a plurality of communication paths are set at the same time between mobile stations and a mobile switching center, and the same communication data is transmitted/received via a plurality of these communication paths. Data having a low error rate is selected, or the same communication data is synthesized with each other on the reception side, so that the data communication with the low error rate is performed. As a consequence, when the channels are switched during the handover, no discontinuity is produced in the channel under use. Thus, no data drops out, and the handover can be realized without any instantaneous interrupt.
However, the above-described prior art technique owns such an initial condition that the data transfer operation in the mobile communication system is carried out in the STM system (synchronous transfer system). For instance, when the synchronization of the frames containing the respective data is maintained in the transfer path between the base stations and the mobile switching center, the frames are transmitted/received only in the specific timing. As a result, the control unit of the mobile switching center merely makes up the relationship between the specific slot and the connection in the transfer path, but need not especially perform the synchronizations in unit of slot and frame during the transmission/reception operations. As a result, even the soft handover is initiated, the data of the respective slots received at the same time, or the specific timing different from each other can be compared with each other and can be selected. Even when the same data is transmitted to each of the plural base stations, if the same data are duplicated and the duplicated data are transmitted by way of the slots for the handover, than the data of the corresponding slot are synthesized, and selected on the side of the base station, or the mobile stations. Thus, the non-instantaneous handover (soft handover) can be readily realized.
On the other hand, one proposal has been made so as to increase the line utilization efficiency by using ATM (asynchronous transfer)-processing having the above-described transfer path and to transfer each of the above-described data by using the ATM cells.
Even when this asynchronous system is employed in the transfer path, there is a first problem that the continuity of the information during the handover must be maintained.
Also, when the above-described transfer path is ATM-processed, as previously described, the relationship between the transmission/reception timing, the connection, and the channel cannot be exclusively specified.
There is a second problem. That is, in the mobile switching center, either the cell or the frame should be identified which contains the same data related to the same handover from a plurality of data received in the mutually different asynchronous timing from the base stations.
Also, there is a third problem. That is, the connection is specified in unit of the cell, or the frame received from the base station, and the judgement is made as to whether or not the handover is initiated in connection therewith. These comparing process and selecting process must be sequentially carried out in unit of the cell, or the frame.
Furthermore, there is a fourth problem. That is, if the handover is initiated, then either the cells or the frames to be transmitted to the base station are duplicated. Thereafter, these duplicated cells or frames should be transmitted at the same time in such timing that the delays and the flickers determined based on the negotiations with the base stations are minimized.
An object of the present invention is to provide such a mobile communication system capable of realizing non-instantaneous interrupt handover even when a transfer path of an asynchronous system is present between mobile stations and a mobile switching center such as the above-described transfer path.
Further, another object of the present invention is to provide a means for identifying either a cell or a frame containing the same data related to the same handover from a plurality of data received from base stations at mutually different asynchronous timing in the mobile switching center in the above-described mobile communication system while non-instantaneous interrupt handover is initiated.
Moreover, another object of the present invention is to provide a means for comparing the respective error rates of the identified cells or frames, for selecting such a cell or frame having the smallest error rates, and for transmitting this cell or frame having the smallest error rate at instructed timing.
A still further object of the present invention is to provide such a means for duplicating data to be transmitted to the base stations, and for transmitting either the cells or the frames containing the duplicated data at the same time in specific timing in the mobile switching center in the above-described mobile communication system while non-instantaneous interrupt handover is initiated.
To achieve the above-described objects, according to an aspect of the present invention, in a mobile communication system arranged by a plurality of mobile stations, a plurality of mobile stations, a plurality of base stations for storing therein these mobile stations, and a mobile switching center for storing therein these stations, either a cell or a frame containing the same data related to the same handover is identified based on header information thereof from a plurality of data received from the base stations at mutually different asynchronous timing; error rates of the identified frames are compared with each other to select the frame having the smallest error rate; this cell or frame having the smallest error rate is transmitted at instructed timing; data which should be transmitted to the base stations are duplicated from information (header information) related to this data; and either cells or frames containing the duplicated data are transmitted at the same time at such transmission timing specified from the header information thereof.
Also, according to another aspect of the present invention, a handover method of a mobile communication system in a wireless communication system constituted by a base station communicated with a plurality of wireless stations, and a mobile switching center connected to a plurality of said base stations, is featured by comprising: a step for initiating a handover process operation in order that while any of the wireless stations is connected to a first base station, the any wireless station should be newly connected to a second base station irrelevant to the first base station; a step such that the wireless station is newly connected to said second base station while keeping the connection with said first base station by the wireless station; a step such that the first and second base stations convert the transmission information from the wireless station into an ATM cell and then transmits the ATM cell to the mobile switching center; a step such that the mobile switching center receives a first ATM cell transmitted by the first base station and a second ATM cell transmitted by the second base station; a step such that the first ATM cell and the second ATM cell, which are received at asynchronous timing, are synthesized with each other; a step for calculating error rates of the first and second ATM cells synchronized with each other; a step for selecting such an ATM cell whose error rate is lower than that of the other ATM cell; and a step such that when such a condition that the error rate of the second ATM cell exceeds the error rate of the first ATM cell becomes a normal condition, the connection between the wireless station and the first base station is cut out.
Also, according to the present invention, the step for synchronizing the first ATM cell with the second ATM cell, which are received in the asynchronous timing, corresponds to a step for temporarily storing the first and second ATM cells received in the asynchronous timing into a buffer so as to be synchronized with each other.
Also, the handover method of the present invention is further comprised of a step for discarding the ATM cell having the higher error rate.
According to another aspect of the present invention, a communication method is featured by comprising: a step such that a plurality of the base stations separately receive information transmitted from the same mobile wireless station in a parallel manner; a step such that the base stations superimpose the received information on an asynchronous frame and transfers the superimposed asynchronous frame to the information processing station; and a step such that when the information derived from the same mobile station is superimposed on the asynchronous frame via the plurality of base stations and then the asynchronous frames are transferred, the information processing station selects an asynchronous frame having a better condition from the asynchronous frames.
Also, according to another aspect of the present invention, a communication method is featured by comprising: a step such that the mobile station adds a frame transmission order to frames to be transmitted; a step such that a plurality of the base stations separately receive information transmitted from the same mobile wireless station in a parallel manner; a step such that the base stations convert the received frame into an asynchronous frame and transfers the converted asynchronous frame to the information processing station; and a step such that when the information derived from the same mobile station is superimposed on the asynchronous frame via the plurality of base stations and then the asynchronous superimposed frames are transferred, the information processing station selects an asynchronous frame having a better condition from the asynchronous superimposed frames to which the same transmission order has been added.
Also, according to the present invention, the same transmission order corresponds to a connection number specifically determined every time a single communication is established.
Also, a communication method, according to another aspect of the invention, is featured by comprising: a step such that the mobile station adds to a frame, information for indicating whether or not a handover process operation is initiated; a step such that a plurality of the base stations separately receive frames transmitted from the same mobile wireless stations in a parallel manner; a step such that the base stations convert the received frames into asynchronous frames and then transfer the converted asynchronous frames to the information processing station; a step for judging information for indicating whether or not the handover process operation is under initiation, which is added to the frame; and a step such that when the judgement result indicates that the handover process operation is under execution, the information processing station causes the asynchronous frames transferred from the same mobile station via the plurality of base stations to be stored into a buffer, selects such an asynchronous frame having a better condition, and outputs the selected asynchronous frame, whereas when the judgement result indicates that the handover process operation is not under execution, the information processing station directly outputs the received asynchronous frame.
Further, a handover method of the invention is featured by comprising: a step for initiating a handover process operation in order that while any of the wireless stations is connected to a first base station, the any wireless station should be newly connected to a second base station irrelevant to the first base station; a step such that the wireless station is newly connected to the second base station while keeping the connection with the first base station by the wireless station; a step such that the mobile switching center duplicates down-stream information to the mobile wireless stations, the number of which is equal to the number of the base stations for executing the handover process operation; converts the duplicated down-stream information into ATM cells; and transmits the ATM cells to the first and second base stations equal to handover sources; a step such that the base station receives the ATM cells transmitted from the mobile switching center and then transmits the received ATM cells to the mobile wireless terminal; and a step for cutting out a connection with any one of the base stations.
Moreover, according to the invention, the mobile switching center is comprised of the steps of: recognizing whether or not a frame containing data to be transmitted to the mobile wireless terminals belongs to a connection of any end-to-end; applying such a number indicative of the same frame transmission order to a frame which is judged as the same connection by the recognizing step; duplicating the frame to which the frame transmission order number has been applied to obtain a plurality of such frames whose number is equal to a total number of the base stations which execute the handover process operation; and synchronizing the duplicated frames with each other and transmitting the synchronized duplicated frames.